Mechanical resonator



April 10, 1962 G. 1.. GRUNDMANN ETAL MECHANICAL RESONATOR Filed March 23, 1959 INVENTORS E usmve L. EBUNDMANN JAMES E. ALBRIEHI if United States Patent 3,028,831 MECHANICAL RESONATQR Gustave L. Grundrnann, Westmont, and James E. Albright, Collingswood, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Mar. 23, 1959, Ser. No. 801,308 8 Claims. (Cl. 116-137) generating ultrasonic'signals of this invention relates to longitudinal mode meof a wide variety of materials, such as metals, glass, ce-

ramics, etc, can be made to mechanically vibrate in a longitudinal mode when shock-excited by a sharp blow from a hammer moving along its longitudinal axis. The mechanical energy delivered to the resonator in this manner subsequently manifests itself as a mechanical vibration having an amplitude which rises sharply to a peak and which decays exponentially'with the passage of time. Such vibrations normally have a fundamental frequency component which is determined primarily by the physical length of the resonator and the propagation velocity of sound in the material from which the resonator is formed. Since most resonator materials do not have a zero temperature coefiicient, and since the frequency generated is a function of the resonator dimensions, the output signal frequency is subject to variation with changes in temperature.

It is an object of this invention to provide an improved ultrasonic wave generator.

Another object of this invention is to provide an improved mechanical resonator which can be excited into longitudinal mode vibration to produce a' resultant signal the frequency of which is substantially independent of variations in temperature.

As pointed out in the copending application of Lucius P. Thomas and Charles C. Iden, entitled, Remote Control System, filed March 6, 1958, Serial No. 719,573, noise interference in ultrasonic remote control systems may be reduced by the simultaneous transmission by a pair of ultrasonic control waves of diflerent frequency. The beat frequency between the transmitted waves is used to derive a control signal for actuating the desired function. Where mechanical resonators were used to generate the ultrasonic waves, this required two resonators for each control function which were simultaneously actuated by ganged striking mechanisms,

it is accordingly a further object of this invention to provide an improved ultrasonic longitudinal mode resonator which may be excited to simultaneously vibrate at two different ultrasonic frequencies.

The resonator of the invention includes a rod comprised of a material which may be excited into longitudinal mode vibration. The resonant rod is centrally supported in a manner to minimize damping of vibrations set up therein. At least one end of the resonant rod is formed to provide two discrete portions of differing longitudinal length as measured from the center of the resonator. When the resonator is excited into vibration, such as for example, by striking one end thereof, vibrations occur at two different ultrasonic frequencies determined by the respective lengths of the two discrete portions. As the dimensions of the resonator are changed with variations in temperaturefthe two frequencies of vibration change in the same direction by substantially similar amounts, and hence the beat "or difference fre- V v In accordance with the invention, the end of the resonator".

quency between the fundamental vibration frequencies remain substantially constant.

The novel features which are considered to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a'perspective view, partly broken away, of an ultrasonic generator including a mechanical resonator embodying the invention;

' FIGURES 2 and 3 are perspective views, partly broken away, of ultrasonic generators including therein modifications of the mechanical resonator shown in FIGURE 1,

and illustrating different means for supporting the resonator; and

FIGURES 4 and 5 are graphs showing waveforms of ultrasonic signals radiated by an ultrasonic generator embodying the invention.

Referring now to the drawings wherein like reference numerals will be used to designate similar elements throughout, and particularly to FIGURE 1, an ultrasonic transmitter 10 is provided which is particularly adapted for remote control applications such as, for example, the remote control of signal receivers, phonographs, etc. The transmitter 10 includes a casing 12 having an opening in one end thereof covered by suitable grillwork 14 which permits supersonic waves to be radiated from the casing. Enclosed within the casing 12 is a cylindrical rod shaped, longitudinal-mode resonator 16 of the invention, which is loosely supported at its center by a bracket 18. The bracket 18 includes a circular aperture 20 therein of slightly larger diameter than the outside diameter of the resonator 16. The resonator 16 is held in the bracket 18 by projections 22 which project radially from the surface of the resonator. Before assembly, the projections 22'are formed by deforming portions of the surface of the resonator circumferentially about the center thereof after which the resonator is inserted in the bracket 18. After insertion, similar projections, not shown, are formed on the resonator to engage the other side of the bracket 18 to prevent longitudinal movement of the resonator 16.

A striking element or hammer 24 is positioned in proximity to, but spaced from one end of the resonator 16. The hammer 24 is supported by a resilient spring member 26 and may be affixed thereto by a screw or other suitable fastening device. As shown in the drawings, the

spring 26 is mounted in a cantilever manner such that the free end thereof extends through the top of the casing 12. The free end of the spring 26 is covered by a cap 28 which may be highly styled. In operation, the cap 28 is plucked, or manually engaged and moved to the right as viewed in FIGURE 1 and released. After release, the resilient spring 26 snaps back toward its initial position. The mechanical energy stored in the hammer 24 causes it to overtravel and sharply strike the end of the resonator 16 to set up vibrations therein. The restoring force of the spring 26 prevents multiple contacts between the hammer 24 and the resonator 16 and thereby avoids damping of the resonator which would reduce the amount of vibratory energy available for radiation.

The mechanical resonator shown in FIGURE l is a cylindrical cross-section, however, other cross-sections, such as a rectangular cross-section, may be used without departing from the scope of the invention. The material used in the resonators preferably comprises a material, such as aluminum, having a low internal damping factor.

16 adjacent the grillwork 14 has been formed to provide two discrete portions of differing longitudinal length as measured from the other extremity of the resonator. This has been done by providing a slot along the longitudinal :axis of the resonator and cutting one of the two legs 30 and 32 thus formed, shorter than the other.

When the resonator 16 is struck by the hammer 24 it vibrates at two different resonant frequencies which are primarily determined by the propagation velocity of sound :in the material from which the resonator is formed, and :is an inverse function of the two lengths of the rod as measured from the ends or the legs 30 and 32 to the other extremity of the rod. There are other factors which have some effect on frequency, such as the homogeneity of the material used in the rod, the positioning of the slot, the accuracy with which the slot is centered throughout its length on the longitudinal axis on the resonator, the shape of the bottom of the slot and the coupling effect between the two portions of the resonator. In the manufacture of a given resonator, the effects of these factors can be determined experimentally to establish the desired dimensional specifications of the resonator. With cylindrical rods it was noted that with the slot on the central axis, slight variations in the radial positioning thereof cause substantial differences in the resulting difference frequency. However, if the slot is purposely machined or otherwise formed about two mils in either direction off center, the difference frequency is substantially unelfected. This effect was not noticed with rods of rectangular crosssection.

Since the resonator vibrates at both the higher and lower frequencies at the same time these frequencies become linearly added in the radiated ultrasonic wave. FIGURE 4 is a graph depicting the radiated output of a resonator in accordance with the invention, which may be observed by using a suitable transducer such as an electrostatic microphone responsive to the radiated ultrasonic frequencies which may be, for example, 38 and 40 kc., respectively, to drive an oscilloscope. It will be noted that the radiated waves are initially relatively large in amplitude and thereafter decrease in amplitude with the passage of time. FIGURE 5 is an expanded portion of the graph of FIGURE 4, and shows the apparent modulation of the transmitted wave. This modulation is the difference frequency between the two waves which are radiated, and may be used as an extremely stable frequency signal source which is substantially insensitive to temperature variations, such as for the control signal for remote control system of the type described in the aforementioned copending application.

The effective percentage of the modulation shown in the graph of FIGURE 5 is primarily determined by the distance between the bottom of the slot and the longitudinul center of the resonator, and by the width of the slot. These dimensions may be adjusted exeprimentally to produce the desired percentage of modulation from the resonator.

Accurate adjustment of the modulation or difference frequency may be obtained by accurate machining of the length of the two portions of the rod, that is, the lengths as measured from the solid extremity to the end of the short leg 32 and the end of the long leg 30, respectively. If the rod is manufactured by using mass production techniques, the tolerances required may not be sufficiently close to enable highly accurate control of the difference frequency of the resonator. However, the difference frequency may be adjusted by removing material from one of the legs 30 and 32 of the resonator 16. For example, if a hole is drilled in the longer leg 30, the frequency difference between the frequencies of vibration of resonator 16 will be decreased. Conversely, if a hole is drilled in the leg 32 of the resonator 16, the frequency difference etween the two modes of vibration will be increased. The drilling of these holes shifts the two frequencies of vibration by only a few hundred cycles, and therefore,

does not move them out of the pass band of a remote receiver with which the transmitter may be used. This means that a method of accurately setting the effective modulation frequency for the resonator hasbeen provided without the necessity of resorting to accurate machining of the length at the rods.

By way of example, a resonator which provides excellent performance was constructed of aluminum alloy #2017. This resonator comprises a cylindrical rod having a diameter of .375 inch, and a length from the solid extremity to the end 30 of 2.626 inches. The length from the solid extremity to the end 32 of the shorter leg is 2.406 inches. The slot is of an inch wide and 1.198 inches deep measured from the end 32, and the rod is supported about 1.2 inches from the solid end thereof.

"Ihe ultrasonic generator shown in FIGURE 2 illustrates a modification of a mechanical resonator constructed in accordance with the invention. The supporting means for the resonator 36 of FIGURE 2 differs from that which is shown in FIGURE 1 in that the resonator '36 has a pair o f'holes drilled therethrough at right angles to each other near the center thereof. Supporting wires 38 and 40 are passed through these holes and may be welded or otherwise afiixed to the bracket 42 to hold the resonator in the desired position. The drilling of the mounting holes alters the frequency of vibration of the resonator and must be taken into account in determining the proper length of the different portions of the rod to obtain a predetermined frequency ofoperation. Accordingly, it is desirable to drill these holes before the rod is processed to form slots in the ends thereof.

The resonator shown in FIGURE 2 is of cylindrical cross-section and as described in connection with El"- URE 1 is formed of a material having a low internal damping factor which may be shock excited to produce longitudinal vibrations. Both ends of the resonator 36 are slotted along the longitudinal axis thereof, and a leg of each end on the same side of a plane passing through the slots is cut short. When the resonator 36 is struck by the hammer 24, vibrations at two different frequencies are established. The factors affecting the frequencies of vibration are the same as those described above in connection with FIGURE 1. It has been determined experimentally that a rod of the same material and length as that shown in FIGURE 1 which is formed on both ends in the same manner as the rod of FIGURE 1 is formed on one end, produces substantially the same frequencics of vibration. A difference in operation results, however, in that the modulation frequency builds up more slowly with the resonator of FIGURE 2. This is because a greater number of reflections of sound energy from one end of the rod to the other are required before sufficient energy is transferred from one portion of the resonator to the other to excite the other mode of oscillation of the resonator. This is because coupling between the two portions of the resonator is reduced since the coupling is effected essentially only in the solid portion of the rod near the center thereof.

\As mentioned above, it is not necessary that the resonator 36 be of cylindrical cross-section be used in the ultrasonic generator of the invention. For example, as shown in FIGURE 3 a rod of rectangular cross-section may be used. The rod or resonator 44 of FIGURE 3 includes slots formed in either end thereof in a manner similar to that shown above in connection with FIGURE 2. However, in the resonator of FIGURE 3 the longer extending legs as measured from the center thereof are on opposite sides of the resonator 44 whereas in FIGURE 2 the longer extending legs of the resonator 36 are both on the same side thereof. In operation, the resonator of FIGURE 3 also vibrates at two different frequencies simultaneously, however a still longer time is required for the modulation component to be established than for the resonator 36 of FIGURE 2. For example, with the resonator 44 of FIGURE 3 a '30 millisecond delay will occur '5 before the modulation component builds up to the maximum level. As mentioned above, this is because a large number of reflections baci and forth between the ends of the resonator 44 are required before suhicient energy is transferred to establish the other mode of vibration.

The resonator id of FIGURE 3 is supported in a dif ferent manner from those shown in FIGURES l and 2. T he support for this resonator 44 comprises a pair of Wires 4s and which extend through spaced parallel apertures drilled transversely through the minor dimension of the resonator i i. The wires 46 and 48 are fastened to a pair of support members 5i? and 52 as shown in FIGURE 3. The means for exciting the resonator 44 is the same as that shown above in that a cantilever spring supports the hammer mechanism and the spring may be plucked to cause the hammer to strike one end of the resonator to excite the vibrations.

What is claimed is:

1. An ultrasonic resonator for vibrating at two fre quencies simultaneously, comprising a resonating rod comprising a material having a low internal damping factor, one end of said rod being formed to provide two discrete and substantially parallel portions of differing longitudinal length as measured from the center of said rod, and means supporting said rod intermediate the ends thereof to permit vibrations to be set up therein in response to shock excitation of said rod.

2. An ultrasonic resonator for vibrating at two frequencies simultaneously, comprising, a rod-shaped memher having a low internal damping factor, at least one end of said member having a pair of longitudinally extending parallel leg portions of diii'erent lengths, and means for supporting said member intermediate of the ends thereof to permit longitudinal mode vibrations to be set up therein in response to shock excitation of said member.

3. An ultrasonic resonator for vibrating at two frequencies simultaneously, comprising, a rod shaped member having a low internal damping factor, both ends of said rod shaped member including a pair of longitudinally extending leg portions of different lengths, and means for supporting said member intermediate of the ends thereof to permit longitudinal mode vibrations to be set up therein in response to shock excitation of said member.

4. An ultrasonic resonator for vibrating at two frequencies simultaneously, comprising, a rod shaped memher having a low internal damping factor, both ends of said rod shaped member including coplanar axially extending slots to form a pair of parallel leg portions on both ends thereof, the leg portions on one side of a plane passing through said slots being longer than the leg portions on the other side thereof, and means for supporting said rods to permit longitudinal mode vibrations to be set up therein in response to shock excitation of said rod.

5. An ultrasonic resonator for vibrating at two frequencies simultaneously, comprising, a rod shaped member having a low internal damping factor, both ends of said rod shaped member including coplanar axially extending slots to form a pair of juxtaposed legs of dilferent lengths on both ends thereof, the longer leg of one end of said rod shaped member being on the opposite side of a plane passing through said slots from the longer leg on the other end thereof, and means for supporting said rods to permit longitudinal mode vibrations to be set up therein in response to shock excitation of said rod.

6. An ultrasonic resonator for vibrating at two frequencies simultaneously, comprising, a rod comprised of a material having a low internal damping factor, at least one end of said resonator being formed to provide two discrete portions of differing longitudinal length as measured from the center of the resonator, and means supporting the rod to permit vibrations to be set up therein in response to shock excitation of said rod comprising a frame member defining an aperture of the same configuration as and slightly larger than a cross-section of said rod discrete portions of differing longitudinal length as measured from the center of the resonator, and means supporting the rod to permit vibrations to be set up therein in response to shock excitation of said rod, comprising a frame member defining an aperture for receiving said rod, means providing a pair of holes extending at right angles through said rod near the center thereof, and

means extending through said holes and afiixed to said frame member for supporting said rod in the desired position.

8. An ultrasonic resonator for vibrating at two frequencies simultaneously, a rod shaped member of rectangular cross-section comprised of a material having a low internal damping factor, both ends of said rod shaped member including coplanar axially extending slots to form a pair of juxtaposed legs of different lengths on both ends thereof, the longer leg of one end of said rod shaped member being on the opposite side of a plane passing through said slots than the longer leg on the other end thereof, and means for supporting said rods to permit longitudinal mode vibrations to be set up therein in response to shock excitation of said rod including a frame member, and a pair of wires passing through a pair of substantially parallel holes in the major surface of said member near the center thereof affixed to said frame member.

References Cited in the file of this patent UNITED STATES PATENTS Wald Feb. 4, 1958 

